[PATCH] D62990: [SCEV][WIP] Try fix PR42175

[Z. Zheng via Phabricator via llvm-commits]

zzheng updated this revision to Diff 203490.
zzheng added a comment.

re-upload with full context


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D62990/new/

https://reviews.llvm.org/D62990

Files:
  include/llvm/Analysis/ScalarEvolution.h
  lib/Analysis/ScalarEvolution.cpp
  test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll


Index: test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
===================================================================
--- test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
+++ test/Analysis/ScalarEvolution/2008-11-18-Stride2.ll
@@ -1,7 +1,7 @@
 ; RUN: opt < %s -analyze -scalar-evolution 2>&1 | FileCheck %s
 
 ; CHECK: Loop %bb: backedge-taken count is ((999 + (-1 * %x)) /u 3)
-; CHECK: Loop %bb: max backedge-taken count is 334
+; CHECK: Loop %bb: max backedge-taken count is 333
 
 
 ; This is a tricky testcase for unsigned wrap detection which ScalarEvolution
Index: lib/Analysis/ScalarEvolution.cpp
===================================================================
--- lib/Analysis/ScalarEvolution.cpp
+++ lib/Analysis/ScalarEvolution.cpp
@@ -10434,8 +10434,8 @@
                                                     const SCEV *Stride,
                                                     const SCEV *End,
                                                     unsigned BitWidth,
-                                                    bool IsSigned) {
-
+                                                    bool IsSigned,
+                                                    const Loop *L) {
   assert(!isKnownNonPositive(Stride) &&
          "Stride is expected strictly positive!");
   // Calculate the maximum backedge count based on the range of values
@@ -10444,6 +10444,12 @@
   APInt MinStart =
       IsSigned ? getSignedRangeMin(Start) : getUnsignedRangeMin(Start);
 
+  // If loop entry is guarded by an ULT, we know (Start-Stride) is in [0, End).
+  if (!IsSigned && !isa<SCEVConstant>(Start) && isLoopInvariant(End, L) &&
+      isLoopEntryGuardedByCond(L, ICmpInst::ICMP_ULT,
+          getMinusSCEV(Start, Stride), End))
+    MinStart += getUnsignedRangeMin(Stride);
+
   APInt StrideForMaxBECount =
       IsSigned ? getSignedRangeMin(Stride) : getUnsignedRangeMin(Stride);
 
@@ -10567,7 +10573,7 @@
   // checked above).
   if (!isLoopInvariant(RHS, L)) {
     const SCEV *MaxBECount = computeMaxBECountForLT(
-        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
     return ExitLimit(getCouldNotCompute() /* ExactNotTaken */, MaxBECount,
                      false /*MaxOrZero*/, Predicates);
   }
@@ -10604,7 +10610,7 @@
     MaxOrZero = true;
   } else {
     MaxBECount = computeMaxBECountForLT(
-        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned);
+        Start, Stride, RHS, getTypeSizeInBits(LHS->getType()), IsSigned, L);
   }
 
   if (isa<SCEVCouldNotCompute>(MaxBECount) &&
Index: include/llvm/Analysis/ScalarEvolution.h
===================================================================
--- include/llvm/Analysis/ScalarEvolution.h
+++ include/llvm/Analysis/ScalarEvolution.h
@@ -1838,7 +1838,7 @@
   /// assert these preconditions so please be careful.
   const SCEV *computeMaxBECountForLT(const SCEV *Start, const SCEV *Stride,
                                      const SCEV *End, unsigned BitWidth,
-                                     bool IsSigned);
+                                     bool IsSigned, const Loop *L);
 
   /// Verify if an linear IV with positive stride can overflow when in a
   /// less-than comparison, knowing the invariant term of the comparison,


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